1. Field of the Invention
The present invention relates to an image processing system and an image forming apparatus incorporating same, and more particularly, to an image processing system that enlarges image size by inserting additional pixels in an original image, and an image forming apparatus incorporating such an image processing system.
2. Discussion of the Background
As electrophotographic technologies have developed to provide high speed, high definition, and high performance image forming systems, modern image data processors are required to cope with various issues arising from the high quality requirements of such electrophotographic systems.
One issue faced by an image data processor employed in an electrophotographic system is the occurrence of errors such as image misalignment or misregistration, which result when multiple images are formed on a single sheet-like recording medium (hereinafter also “recording sheet”).
Image misalignment takes place in printers featuring an automatic duplex mode, where images are successively created and melted on to both sides of a single recording sheet. As a recording sheet tends to shrink slightly when heated, heat application between successive passes causes slight but noticeable misalignment of the images printed on opposite sides of the same sheet. Such a defect seems to issue from the faster processing speeds and compactness typical of modern electrophotographic printers, which reduces a time or a space interval between successive image formations in the duplex printing mode.
Image misregistration occurs where a full color image is produced by superimposition of sub-images of distinct colors formed by multiple electrophotographic units. Such errors are commonly encountered by high speed color printers that employ a tandem architecture for reducing processing time.
One solution to such misalignment and misregistration defects is to use a scaling function, which adjusts image size by electrophotographically adding or deleting picture elements (pixels) to or from an original image before creating a final image.
FIG. 1 is a schematic illustration of an example of scaling operation where image size is enlarged by adding new pixels.
In FIG. 1, two-dimensional arrays 600 and 610 are corresponding portions of pixel matrices or image data before and after a scaling operation, where each square cell represents one pixel, “X” or horizontal direction indicates a main scan direction, and “Y” or vertical direction indicates a sub-scan direction.
In the scaling operation, a single row of additional pixels is inserted after every ten rows or scan lines of the image data 600 to shift the original pixels in the sub-scan direction Y. This obtains the resulting image data 610 vertically enlarged, with the rows of added pixels 612 periodically appearing in the pixel matrix. When used in electrophotographic printers, such a scaling function enables fine adjustment of image size in a desired direction, thereby effectively preventing misalignment or misregistration.
A drawback of such a size adjusting method is that the scaled image tends to suffer from an undesirable pattern of streaks, a print defect commonly known as “banding”.
Generally, artifacts on a printed image such as banding and moiré patterns result from periodic density variations due to various causes, including periodic insertion or deletion of pixels in the image data. As the human eye is most sensitive to repeating parallel lines with spatial frequencies ranging from 5 to 10 cycles per degree (cpd), equivalent to 0.8 to 1.6 cycles per millimeter (cpm) at a viewing distance of 350 millimeters, density variations periodically appearing at such frequencies are perceived as a defect on a printed image.
In the case of the scaling operation depicted in FIG. 1, an image can be scaled up to 102% by inserting one scan line after every 48 scan lines. However, when the image has a resolution of 1,200 dots per inch, such pixel insertions translate into density variations in the printed image occurring at approximately 1 cpm, which falls within the range of perception to cause a noticeable banding defect.
Consequently, it is desirable to compensate for periodic variations when processing image data for output to an imaging system, and for this purpose, various techniques have been proposed in the field of image data processing.
One known method provides an electrophotographic color image forming apparatus with an offset detector and a compensation circuit. When a full color image is created by superimposition of multiple sub-images originating from different imaging stations, the offset detector detects an amount of displacement for each sub-image using a reference pattern. Based on the offset information derived from the offset detector, the compensation circuit calculates a number of pixels to be added to compensate for the displacement amount. Pixels are randomly inserted in image data by the calculated amount, so that the multiple sub-images have substantially the same width along a scan direction. According to this method, a defect-free color image may be obtained with no periodic variation present in any direction.
Another known method proposes a color image forming apparatus with a registration capability, where a screening unit processes image data with a halftone screen and a registration unit serves to compensate for misregistration by adding or deleting a given sequence of pixels in the processed image data. When the screening unit applies halftone screens of different frequencies to different portions of an original image, the registration unit correspondingly adjusts the frequency at which to insert or delete pixels so that the screen frequency and the insertion/deletion frequency do not coincide with each other at least within a range of 0.5 millimeters in the resulting image.
Unfortunately, these conventional methods require complex calculations and significant memory capacity in determining where to insert or delete pixels in the image data, increasing manufacturing cost of a printing system implementing the size adjustment capability.
Therefore, it would be advantageous to have a cost-effective, low memory consumption image processor for use in electrophotographic systems, capable of adjusting image size without causing significant loss of image quality. An electrophotographic image forming apparatus incorporating such an image processor can provide high quality images with enhanced performance and reduced manufacturing cost.